The Dark Side of Lead-Free Metal Halide Nanocrystals: Substituent-Modulated Photocatalytic Activity in Benzyl Bromide Reduction

We illustrate here the high photocatalytic activity of sustainable lead-free metal halide nanocrystals (NCs), namely, Cs3Sb2Br9 NCs, in the reduction of p-substituted benzyl bromides in the absence of a cocatalyst. The electronic properties of the benzyl bromide substituents and the substrate affinity to the NC surface determine the selectivity in C–C homocoupling under visible light irradiation. This photocatalyst can be reused for at least three cycles and preserves its good performance with a turnover number of ca. 105,000.


Synthesis of Cs3Sb2Br9 NCs.
Colloidal Cs3Sb2Br9 NCs, were synthesized by hot-injection approach. In a typical synthesis, Cs2CO3 (0.25 mmol; 81.45 mg), Sb(CH3CO2)3 (0.331 mmol; 99.06 mg), 4 mL of ODE, 0.6 mL of OLA, and 1.2 mL of OA were mixed in a 50 mL 3-necked round-bottom flask inside a nitrogen glovebox and then, the mixture was heated up to 140 °C under vacuum for 1 h. Afterward, a benzoyl bromide dispersion (210 μL/0.5 mL) in degassed ODE was swiftly injected inside the flask under N2 atmosphere. Then, the reaction was immediately quenched in an ice−water bath and 4 mL of ethyl acetate was added to the crude NCs solution and centrifuged at 5500 rpm for 5 min. The final precipitate was dispersed in toluene (4 mL) and stored in a glove box for further use. All the washing procedures were carried out under an inert atmosphere.

General procedure for the photocatalytic reduction of benzyl bromides.
The photocatalytic reactions were prepared in a nitrogen glovebox and conducted in 10 mL gastight crimped vials under stirring (160 rpm) using an orbital shaker in a photoreactor with blue LEDs (405 nm) for 48 h at 30 °C. To prepare the reactions, 450 μl of a stock solution of the colloidal Cs3Sb2Br9 NC dispersion was added to a vial, dried over the vacuum, and weighted for every single reaction (10.0  0.5 mg). Then, inside the glove box, the substrate (100 μl of the psubstituted benzyl bromide stock solution, final concentration 17 mM), the electron donor (MeOH, 20 equivalents), and the corresponding solvent (VF=2 mL) were added. Immediately after the reaction finished, the crude of the reaction was centrifuged at 12500 rpm for 15 minutes, and then, biphenyl (75mM) was added as the internal standard (IS) to the supernatant and subjected to GC-MS analysis to determine the conversion of substrates and the yield of the desired product. Both substrates and biphenyl were dissolved in ethyl acetate for those reactions performed in hexane, due to their low solubility in this solvent.
The reported yield of the product obtained was an average of at least 2-5 runs. The pellet was redispersed in toluene to recover the colloidal perovskite after the photocatalytic cycle.

Photocatalytic system.
Light source: The reactions were performed using (λ= 405±10 nm) LUXEON LED, mounted on a 10mm Square Saber -1030 mW@700mA as a light source.
Temperature Control: Reaction temperature was controlled by a high-precision thermoregulation Hubber K6 cryostat. Likewise, to guarantee stable irradiation the temperature of the LEDs was set up at 21 °C.
The reactions have been carried out in an in-house parallel High Throughput Screening (HTS) photoreactor with a capacity to set up to 25 reactions with different excitation wavelengths, respectively, under high-intensity irradiation. These unique HTS platforms allow for tight control of the light intensity and the temperature of the reactions. The 25-positions photoreactor is operable at 1-15 mL reaction volumes for each reaction.
Photograph 1. In-house developed 25-well parallel photoreactor used for the reactions.

Optical Characterization.
The absorption spectra of the NCs dispersion were recorded using a Cary300 spectrophotometer and UV/VIS/NIR spectrophotometer Lambda 1050, equipped with software PerkinElmer UV Winlab-ink. The absorption spectra of the NCs in films were measured using a 150 mm InGaAs Integrating Spheres Module of Lambda 1050, equipped with software PerkinElmer UV Winlabink. The samples were prepared by diluting NCs samples in 2 mL of toluene in 1 cm path length quartz cuvettes with airtight caps. The NCs were prepared in films, by drop-casting 100 μL of NCs suspensions (5 mg/mL) over the center of a glass substrate (75x75 mm).

Thermogravimetric analysis (TGA).
Thermogravimetric analysis of the NCs was carried out with a Mettler Toledo TGA/SDTA851e/SF/1100 apparatus in the 25-800 °C temperature range under a 10 °C min −1 scan rate in a nitrogen atmosphere.

X-ray Diffraction (XRD) Characterization.
XRD patterns of the NCs were acquired with a Pananalytical Empyrean X-ray diffractometer equipped with a 1.8 kW Cu Kα ceramic X-ray tube and a PIXcel3D 2 × 2 area detector, operating at 45 kV and 40 mA. Specimens for XRD measurements were prepared by dropping a concentrated NC dispersion onto a silicon zero-diffraction single crystal substrate. The diffraction patterns were collected under ambient conditions using a parallel beam geometry and the symmetric reflection mode. After reactions, the XRD patterns were measured with a powder diffractometer Empyrean from Panalytical equipped with CuKα anode operated at 45 kV and 40 mA. Single scans were acquired in the 2θ=5° to 60° range with a step size of 2θ=0.01° in Bragg-Brentano geometry in air. XRD data analysis was conducted using the HighScore 4.1 software from Panalytical.

X-ray Photoelectron Spectroscopy (XPS) Characterization.
XPS spectra were collected on the sample before and after the photoreduction of p-Br-benzyl bromide 4a performed in toluene or in hexane. Few microliters of the samples' solutions were drop cast of clean Au substrates. XPS data were acquired using a Kratos Axis UltraDLD spectrometer, using a monochromatic Al Kα source, operated at 20 mA and 15 kV. High resolution XPS scans, reported in figure S26, were acquired at pass energy of 20 eV, with an energy step of 0.1 eV, over an area of 300 x 700 microns.
The Kratos charge neutralization system was used during data acquisition. The binding energy scale was calibrated by setting the main line of the carbon 1s spectrum to 284.8 eV. Spectra were analysed using CasaXPS software (version 2.3.24).

Attenuated total reflectance-Fourier transform infrared spectroscopy
To ascertain the substrate approach to the Cs3Sb2Br9 NC surface, the NCs and the substrates were mixed at the same concentration used in the photocatalytic reactions and the mixture was stirred for several hours in an orbital shaker to ensure the interaction between them. Then, the ATR-FTIR analyses were performed in a Bruker alpha II spectrometer by drop-casting 20 μL (2x10 μL) of the colloidal mixtures.

Nuclear magnetic resonance spectroscopy
The interaction of benzyl bromide with the surface of Cs3Sb2Br9 NCs was also monitored by 1 Hand 13 C-NMR. A colloidal dispersion of the NCs and the substrate was prepared in toluene-d8 at the same ratio as in the photocatalytic reactions. Then, the colloidal dispersion was stirred in an orbital shaker for several hours and measured in a Bruker AV400 (400 MHz).

Gas chromatography-mass spectrometry
The work-up of the reaction was performed by adding the biphenyl (IS) to the crude. Then, 0.1 mL of the mixture was diluted with 0.9 mL of anhydrous toluene or hexane. The solution was injected into the GC and the products were identified according to the retention time and mass GC-MS (Agilent 7890B -5977A).
A calibration curve was carried out using different concentrations of analytes (CAN) in comparison to a given concentration of the internal standard (CIS=75 mM). Then, it was analyzed the GC signals to compare the areas between analytes and internal standard generating a plot with a precise correlation. The coefficient between the area of the products and the IS used, allows us to quantify the product concentration of the reactions. This plot was used to study the products obtained.
Correlation between the GC areas of the analytes / internal standard and their concentration.
The following equations were used to calculate the product yield (1) and substrate conversion (%; 2):

Calculation of the photocatalyst active amount (mol %), turnover number (TON) and turnover frequency (TOF)
To calculate the amount (in mol %) of Cs3Sb2Br9 NCs used for the photoreactions, it was followed the method used by Angshuman Nag et al. 1 for calculating the mol of CsPbBr3 NCs and the molar extinction of this material. Firstly, a TGA of the NC dispersion was done in order to obtain the inorganic part of the photocatalytic system (Cs3Sb2Br9 NCs + organic ligands + ODE). Thus, if it was weighted 10 mg of the photocatalytic system dried for every single reaction, only 2.5 mg (25 % of the material) corresponded to photoactive Cs3Sb2Br9 NCs. Then, considering the round shape of a single NC and its diameter (18.3 nm), calculated by the Scherrer equation, was determined the weight of a single NC using the formula (1), which takes into account the density of the material (3.97 g/cm 3 ) 2, 3 and the volume of a nanosphere [v=(4/3) x  x r 3 )]. The number of NCs in the photoreaction batch was determined by dividing the inorganic part of the NCs weighted by the weight of a single NC (2) and for the mol of NCs (3), was divided the number of NCs by Avogadro's number (NA). Finally, the mol % of photocatalyst was obtained using the formula (4).
(1)  For the analysis of the material photoactivity was calculated the TON and TOF following the equation (5) and (6):

Modified Scherrer equation
The modified Scherrer equation method 4 has been used to get accurate NCs size, using the XRD pattern. The purpose of the modified Scherrer equation is to minimize the error of this equation, when the diameter of the NCs is measured at different XRD angles, using a leastsquares technique. For that, it's plotted ( ) (y -axis) against ( θ ) (x -axis) obtaining an intercept of a least-squares line regression, which is referred to the diameter of the NCs.
For that, Scherrer equation (7) is modified to (8) Then, the modified Scherrer equation could be plotted as a straight line (y=mx+c), and a leastsquare line regression is performed to calculate the grain size from the y-intercept " = ln ( ) " value.

Control experiments of the photocatalytic reduction of benzyl bromides
All the reactions have been performed in the same conditions reported for the photocatalytic reactions but in the presence of the scavengers. Regarding the control experiments, 0.34 mmol of TEMPO and AgNO3 were added as scavengers, whereas the last reactions were performed in a normal air atmosphere. To avoid the presence of radicals produced after the toluene oxidation, all the reactions experiments were carried out in hexane and using Cl-benzyl bromide as the substrate to directly monitor the formation of the C-C coupling products.

Calculation of the photocatalytic activity in the recycling experiments
Recycling experiments were performed by triplicate. The NC dispersions were centrifuged after each photocatalytic cycle and the precipitate was reused in the next cycle. In this process, some amount of the photocatalyst was lost due to its adsorption to the glass reactor and/or during the precipitation step. To exclude the influence of the photocatalyst mass, the photocatalytic activity was calculated as follows: the product yield was divided by the photocatalyst mass for every single reaction and cycle, and then normalized by the maximum product yield (100%) and the initial amount of photocatalyst (10.5 mg in these reactions), using equation 9. As has been depicted in Figure S27a, the photocatalytic activity of the NCs remained very similar after the 3 cycles with a standard deviation between the different cycles of 5 %.
The samples were prepared by dropping dilute NC solutions onto carbon-coated 200 mesh copper grids. Low-resolution TEM analyses were performed on a HITACHI HT7800 microscope with a filament of LaB6 operated at 100 keV.

High resolution transmission Electron Microscopy (HRTEM) Analysis.
High-resolution TEM (HRTEM) images were taken on a Field Emission Gun (FEG) TECNAI G 2 F20 microscope operated at 200 kV. The samples were prepared by drop-casting few drops of a toluene NCs dispersion onto a carbon film supported on a copper grid, which was subsequently dried under vacuum before the examination.

Scanning Electron Microscopy and Energy-dispersive spectroscopy (SEM-EDS).
SEM-EDS analysis was performed on a HRSEM JEOL JSM-7500LA microscope with a cold fieldemission gun (FEG), operating at 15 kV acceleration voltage. Energy-dispersive spectroscopy (EDS, Oxford instrument, X-Max, 80 mm2) was used to evaluate the elemental ratios. All experiments were done at an 8 mm working distance, 15 kV acceleration voltage, and 15 sweep count for each sample. SEM images were taken using a Hitachi S-4800, operating at 20 kV acceleration voltage.

Electrochemical properties of Cs3Sb2Br9
Redox properties characterization was performed on an Autolab potentiostat (Autolab 128N potentiostat/galvanostat) using a three Note that if the oxidation potential of the Cs3Sb2Br9 material is taken as 1 V (second irreversible peak at positive CV values), the electrochemical gap (2.5 eV) coincides approximately with the value of the optical gap (2.58 eV). This suggests that this is an acceptable approach for estimating energy levels. This methodology is widely used in literature to determine the band gap of semiconductors. 5   Figure S1. a-b) Representative SEM images of the spheric Cs3Sb2Br9 NCs. c) HRTEM of one Cs3Sb2Br9 NC overlapped with its inverted Fourier transformation (scale bar 10 nm) and d) intense height profile of the planes measured disclosing the presence of an interplanar distance of 0.198 Å which is ascribed to the (204) crystalline plane.

Products characterization
For product characterization, gas chromatography coupled with mass spectrometry was used. In these studies, biphenyl (75 mM) was employed as an internal standard. Table S3 summarizes the retention time of the different analytes observed. Figure S6. Gas chromatography of the benzyl bromide reaction in toluene and mass spectra of the biphenyl internal standard and bibenzyl product.

3.5.
Mechanistic studies of the photocatalytic process Figure S17. Correlation between the product yield of the C-C homocoupling product (a) and that of the dehalogenation product (b) with the sigma value (p) for different p-substituted benzyl bromides.

Br2 detection experiments
With the aim of ascertaining if the Br2 content could come from the leaching of the structural Br from Cs3Sb2Br9 NCs under the photocatalytic reaction conditions, control experiments were carried out to study the Br2 formation. The reaction was performed in the presence and the absence of benzyl bromide (1a), preserving the same photocatalytic conditions as those used in all the reactions performed before. As can be observed in Figure 5, the supernatant of the reaction in the presence of substrate presented a brownish color and the absorption spectra showed the typical band (λabs. max.=324 nm) ascribed to the charge-transfer complex between the Br2 and aromatic compounds. However, when no substrates were added to the reaction, the supernatant was colorless, thus ruling out the possible loss of Br-from the NC surface. The addition of cyclohexene in carbon tetrachloride (CCl4) to the brown-yellowish supernatant of the p-OMe-benzyl bromide reaction did not produce any discoloration associated with the addition of the Br2 to the double bound. [6][7][8] As, the direct measurement of molecular Br2 was not possible because it formed a charge transfer complex with the aromatic compounds (substrate and products), as it has been previously reported for Br2-benzene complexes (absorption band ca. 300 nm, Figure S16). [9][10][11] Two additional experiments were done to confirm the presence of molecular Br2: i) the addition of KOH to break down the charge transfer complexes, followed by UV-light irradiation to deliver molecular Br2 ( Figure S17a, inset molecular Br2 spectrum) and ii) irradiation of the charge transfer complex (ex=365 nm during 90 minutes), thus eventually leading to the substitution of H by Br with the concomitant discoloration of the sample ( Figure  S17b). The photoaddition of Br2 to aromatic compounds has been reported for bromobenzenes and p-substituted toluene derivatives. 12